In many applications, it is desirable to have a plastic composition which is transparent, has antistatic or electrostatic dissipative (ESD) properties and enhanced physical properties, such as impact strength. Transparency is a particularly desirable property for packaging materials and containers to permit visual inspection of the packaged contents. However, typical prior art transparent plastic materials having low conductivity often accumulate static electrical charges during formation and use. The presence of a static electrical charge may cause dust to adhere to articles packaged with such plastic material or it may cause the plastic material to adhere to itself or other articles.
Electrodissipative materials prevent the buildup of such static charges. The accumulation of static electrical charges upon plastic material may be prevented by incorporating within or coating the surface of the plastic material with an antistatic agent. For example, electronic components are often shipped in transparent, extruded plastic tubes which are treated with antistatic agents to inhibit the formation of static electrical charges which may damage the components. The transparent tubes permit visual inspection of the tube content for product identification.
In order to achieve antistatic behavior by blending an inherently anti-static polymer with another resin it is important that the two resins are dispersed uniformly. However, it is equally important that there be two distinct polymer phases so that the inherently antistatic polymer will dissipate the charge at the surface of the molded article.
In order to achieve transparency one skilled in the art would be led to select immiscible polymers, one of which is antistatic in nature, which are reasonably compatible to achieve a uniform dispersion and finally which are identical in refractive index to achieve transparency.
Unfortunately, many prior art inherently antistatic polymers have refractive indices sufficiently different from the refractive index of the described base polymer so as to adversely affect transparency. In almost all such prior uses the refractive indices will be only those providing some degree of translucency, at best. It is at this point that, in order to achieve transparency, a more unique blend is needed.
Alternate approaches have been tried, as mentioned above, using soluble additives or coatings. However, the antistatic property of materials including nonpolymeric additives or coatings may deteriorate or become inconsistent if the coating or any additive which blooms to the surface of the material is rubbed away. Also, many antistatic additives are damaged or destroyed by high processing temperatures typically encountered in thermoplastic processing.
It would be desirable to have a transparent, electrodissipative thermoplastic material with high impact strength in which the electrodissipative property is consistent throughout the material and does not appreciably diminish over time.